1. Field of the Invention
This invention relates to a pneumatic ABS system and more particularly to a unique ABS modulator solenoid valve, with a pressure balancing piston, which includes the build, hold and exhaust functions.
2. Description of Prior Art
In heavy vehicle air brake systems a brake valve controlled by the vehicle operator supplies compressed air at a pressure of 10 psi to 120 psi for operating the vehicle service brakes. During braking the air is supplied through the brake valve to brake chambers for operating the service brakes. The pressure of the air supplied to the service brakes is a function of the position of the brake valve which is controlled by the vehicle operator.
Standard practice in the pneumatic ABS industry is to use a modulator between the brake valve and the brake chambers for controlling the air supplied to the brake chambers. Normally the modulator has two separate control circuits using solenoids, one for building pressure in the brake chambers and one for exhausting pressurized air from the brake chambers. The hold function is also accomplished with these two circuits. During the hold function the modulator holds or regulates the air pressure in the brake chambers to a pressure between 0 psi and the pressure supplied by the brake valve. The pressure supplied by the brake valve can vary from 10 psi to 120 psi.
The AlliedSignal M-21 and M-22 antilock system modulators are exemplary of prior art ABS modulators. These modulators are essentially high capacity, on/off air valves that incorporate a pair of electrical solenoids for control. The solenoids provide the electro-pneumatic interface or link between the antilock controller electronics and the air brake system. The modulator includes a normally open exhaust solenoid and a normally closed supply solenoid, an inlet diaphragm valve and an exhaust diaphragm valve. During normal non-antilock operation both solenoids are de-energized. Brake application air enters the supply port and flows through the open exhaust solenoid to the exhaust diaphragm, which with a spring keeps the exhaust port closed. Simultaneously, application air flows to the supply diaphragm, forcing it away from its seat and permitting air flow out the delivery port to the service brake chambers. When the supply solenoid is energized the air flows therethrough closing the inlet diaphragm to prevent communication between the inlet or supply port and the outlet or delivery port. When the exhaust solenoid is energized air flow opens the exhaust diaphragm permitting free communication between the exhaust port and the delivery port for rapid exhaust of the compressed air in the brake chamber.
If a service brake application is made and the antilock system detects an impending wheel lockup, the antilock controller will immediately begin modification of the brake application using the modulator. In order to modify the brake application, the coils of the two solenoid valves contained in the modulator are energized or de-energized in a pre-programmed sequence by the antilock controller. The solenoids in the modulator are controlled independently by the antilock controller. When a solenoid is energized it either opens or closes thereby causing the exhaust or re-application of air pressure to the brake actuator.
In EBS systems it is know to use a proportional modulator valve with a single solenoid for supplying compressed air at various pressures to the vehicle brake chambers. Compressed air at a set pressure, usually 120 psi, is supplied to the valve inlet port and the pressure at the valve outlet port is controlled to vary directly with the current applied to the coil of the solenoid which controls the proportional modulator valve. U.S. Pat. No. 5,154,203 and German Offenlegungsschrift DE 3,111,716 A1 describe such proportional modulator valves.